This invention relates to fasteners, methods of manufacturing fasteners, and apparatuses used to manufacture fasteners.
The venerable threaded fasteners consist of a nut and a bolt. The nut has internal threads that thread onto external threads of the bolt. Surfaces of the nut and bolt are formed to receive wrenches which are operated to tightly join the fastener members and one or more pieces together. Broadly, another name for a bolt is a threaded pin, and another name for a nut is a collar.
Many environments in which fasteners are used require the fasteners to have extremely high integrity and strength. Fasteners must bear loads both along a longitudinal axis and radially on the axis. More particularly, when fasteners join together two or more sheets and the sheets are loaded in their planes with different loads, one sheet tends to slide relative to the other. Fasteners passing through both sheets become loaded in shear to prevent the sheets from sliding relative to each other. Axial loads arise from clamping sheets between a head of the pin on one side of the sheets and the collar on the other side of the sheets.
Frequently, fasteners are required to function well in environments where they are cyclically stressed under conditions that could give rise to fatigue failure. A fastener holding two sheets together with an acceptable axial load resists fatigue failure.
An obviously desirable feature of a fasteners is that it does not loosen, fail, or otherwise come apart in service. Many different devices have been used to keep collars and pins together. One way of locking the collar and pin is to deform the threads of the collar so that they bear in radial compression against the threads of the pin. In this method, the resistance to unthreading is purely frictional. The threads are commonly deformed at a manufacturing facility in preference to the field, but field deformation has also been practiced.
It is also highly desirable to know and control the axial load that the fastener is subject to when holding the sheet together. The axial load correlates to the resistance of a collar to further threading onto a pin. As the resistance to further threading increases, the axial load increases, and the torque required to turn the collar increases. These relationships have been used in developed fasteners to provide predetermined axial loads.
In one prior art fastener, a section of a collar adapted to receive a wrench is attached to the main part of the collar by a frangible break neck that breaks upon the application of a predetermined torque corresponding to the desired axial load. The features of a deformed thread lock and a collar with a frangible break neck for axial load control have been combined in one collar. Regrettably, the combination has shortcomings. A thread lock obtained by deformation of the collar threads to form a thread lock, is performed at the factory or in the field before the collar is threaded onto the pin. Thus, the collar does not freely thread onto the pin. This makes threading the collar onto the pin somewhat difficult. Protective and lubricative coatings applied to the threads of the collar to aid threading can be worn off of a collar having this type of thread lock by considerable frictional drag between the threads of the collar and pin. Where corrosion control is important, a circular band of bare metal on the collar is created by failure of the break neck. This band is not protected by corrosion inhibitors applied to the fastener when it was manufactured.
Further, the separation of the collar into two pieces presents several problems. The fact that the section adapted to receive the wrench separates from the threaded section of the collar creates a spare piece that must be removed from the environment where the fastener is attached. This type of fastener is also comparatively expensive because it is difficult to manufacture and requires considerable machining in its formation. The frangible break neck section needs to have very close dimensional tolerances if small variations in break off torques are required. This problem is compounded by machine tool wear in the tools that make the part and also because the break neck section becomes elliptically shaped after the collar threads are deformed to incorporate the thread locking feature into the fastener. Also, the frictional drag between the shear pin and the collar in a fastener system employing a pre-existing deformed thread lock results in a broad range of axial pin loads. The broad range of loads occurs because the drag created by the deformed threads varies greatly among fasteners and is a significant component in the resistance that effects the failure of the frangible break neck. Therefore, the axial load created by the set amount of torque at which the frangible break neck fails varies greatly because of the difficulty in controlling the exact amount of the resistance between the deformed collar and the pin.
A second approach to a locking system employs a pin having an outer groove adapted to receive a deformed collar material. The collar is threaded onto the pin to develop desired axial load, and is then deformed radially inward into the groove so that the deformed collar material is restrained by the walls of the groove and establishes interference. The groove can be made longitudinally or annularly. In one type of such fastener a collar is threaded onto a pin with one setting tool. A second setting tool radially deforms the collar into threads of the pin to effect the interference lock.
To solve these problems, U.S. Pat. Nos. 4,260,005, 4,383,353, and 4,544,312 all to Stencel, the disclosures of which are fully incorporated herein by reference, disclose an improved collar and pin. The collar has external lobes capable of deformation. The deformation of the lobes displaces collar material into an axial bore of the collar, so that the displaced material is brought into interference with a surface of the pin to produce a rotational lock.
The pin has a longitudinally extending shank attached to a head and a plurality of short rounded grooves or flutes extending longitudinally along the shank. Each flute is concave outward in cross section. At the end of the shank opposite the head, roll formed threads cross over the flutes to receive the internal threads of the collar. While being threaded onto the pin, the longitudinally extending lobes on the outer surface of the collar act as a wrenching surface to which a tightening tangential load is applied. Once a determined tangential load or torque is applied, the lobes fail plastically in radial compression. When the lobes plastically deform, material located inwardly from the lobes plastically displaces into the flutes establishing a locking relationship between the collar and the pin created by the interfering material.
To form the pin, the shank of the pin is given a preformed configuration such as that of a generally regular polygon, specifically hexagonal or pentagonal. The corners between the flat sides are rounded. The shank of the pin is then roll formed to introduce threads onto the shank which intersect the flat sides of the shank. The roll forming process deforms material to form the flutes from the flats. The crests of the flutes are formed by radially outwardly displaced shank material formed during the roll forming.
On the collar, each lobe has a convex curvature in radial planes, and the curvatures of the lobes are equal to each other. To make it easy to install a driver onto the collar for tightening, it is preferred that the lobes be located at equally angular distances from each other for example, 120.degree. apart. A driver bears against the lobes with a tangential component force and a component of force in the direction of the axis of the collar.
The collar threads freely onto the work piece of the pin until the collar engages the work piece. Thereupon resistance builds up until the lobes fail inwardly under radial compression. Failure occurs in just a few degrees of arc, and therefore, the amount of axial load on the structure being fastened is determined accurately with small variations. With the failure of the lobes, the setting driver turns freely on the collar indicating that the fastener system is set. Thus, the axial load is controlled without throw-away pieces. Corrosion inhibitors and lubricants are not affected by this deformation.
Though the fasteners of the above patents are made relatively inexpensively, the production of the pins for these fasteners has encountered some inefficiencies. The preforming of the pin shank is a relatively expensive process requiring the fabrication of a cylindrical pin shape and the milling of the desired number of flats onto an end of the pin shank. The precision milling process, which is necessary to produce the preformed pins, is both labor and time intensive. This increases the cost of the pins. After the flats are formed onto the pin, the pin is thread rolled to introduce the threads onto the pin. The flats tend to cause the pins to wobble and otherwise rotate irregularly between the thread rolling dies used in the roll forming process. The irregular rotation of the pins in the dies results in a relatively high scrap rate and increased cost for the pins.
Thus, reduction in the required labor and time for fabricating the pins is desirable to increase the production rate and reduce the production cost of the pins. It is also desirable to reduce the amount of scrap produced during the roll forming process thereby decreasing the cost of producing the pins.